(a) Field of the invention
The present invention relates to an objective lens system for an endoscope and, more particularly, to an objective lens system to be used with a flexible-type endoscope employing an optical fiber bundle as an image transmission optical system or an inflexible-type endoscope employing relay lenses as an image transmission optical system, said objective lens system for an endoscope being especially arranged that the angle of view is wide and distortion and curvature of field are corrected favourably.
(b) Description of the prior art
As a conventional objective lens system for an endoscope of the retrofocus type as shown in FIG. 1, there has already been known, for example, the one disclosed by Japanese published unexamined patent application No. 121547/74. The above-mentioned known objective lens system for an endoscope comprises a lens group I having negative refractive power and a lens group II having positive refractive power which are arranged on the object side and image side with a stop S interposed therebetween. Said known objective lens system is so designed as to obtain a wide angle of view by strongly refracting the principal ray P by means of the lens group I located in front of the stop S and having negative refractive power and, furthermore, arranged that the lens group II located in rear of the stop S and having positive refractive power functions so that the principal ray P, which is incident on the image surface, is made parallel with the optical axis and enters the image guide G in that state. By making the principal ray P incident perpendicularly on the end surface of the image guide G as described in the above, it is possible to reduce the loss of light in the image guide G. Besides, when applying said known objective lens system to an inflexible-type endoscope which employs relay lenses, the principal ray P should be made perpendicular to the image surface O' as shown in FIG. 2 so as to reduce the loss of light in the relay lenses.
As described in the above, said known retrofocus-type objective lens system for an endoscope satisfies two requirements for objective lens systems for endoscopes, i.e., to make the angle of view wide and to make the principal ray, which is incident on the image surface, perpendicular to the image surface.
However, said known retrofocus-type objective lens system for an endoscope still has a disadvantage that negative distortion is remarkable. For example, in the objective lens system for an endoscope shown in FIG. 1, distortion is 21% at .omega.=37.degree. (2.omega.=angle of view). Besides, in cases of other known retrofocus-type objective lens systems for endoscopes, the relationship between the angle of view and distortion is as shown in the table below, i.e., negative distortion is remarkable.
______________________________________ .omega. 20.degree. 30.degree. 40.degree. 50.degree. 60.degree. distortion -6% -13.5% -23% -36% -50% ______________________________________
In order to correct the above-mentioned negative distortion, it is contrived to provide an aspherical surface in the objective lens system. The lens system disclosed by Japanese published unexamined patent application No. 173810/82 is known as an objective lens system for an endoscope having distortion and other aberrations corrected comparatively favourably by using an aspherical surface. However, in spite of the fact that said known objective lens system for an endoscope has a narrow angle of view (2.omega.), i.e., 56.degree., distortion thereof is not corrected satisfactorily favourably.